Fix c-in-knr-argdecl to avoid false recognition of K&R

Fix c-in-knr-argdecl to avoid false recognition of K&R

In the OP's test case, the type of a declaration was a macro
with parentheses, which confused c-in-knr-argdecl.

* lisp/progmodes/cc-engine.el (c-in-knr-argdecl): Amend this
function to return nil if a "parameter declaration" can't be
parsed as a declaration.